Derivatives of aryl ketones as visible sensitizers of photopolymerizable compositions

A photopolymerizable composition comprising the components:

• (i) at least one nongaseous ethylenically unsaturated compound capable of forming a high molecular weight polymer by photoinitiated addition polymerization;
• (ii) at least one 2,4,5-triarylimidazolyl dimer initiator consisting of two 2,4,5-triarylimidazolyl radicals bound together by a single covalent bond;
• and
• (iii) at least one .alpha.,.beta.-unsaturated ketone sensitizer having the formula ##STR11## wherein R.sup.1 and R.sup.2 are each H, or R.sup.1 +R.sup.2 is --CH.sub.2 --;
• R.sup.3 is H, or R.sup.3 +R.sup.5 is selected from --CH.sub.2 --.sub.2, --CH.sub.2 --.sub.3, and --CH(CH.sub.3)CH.sub.2 C(CH.sub.3).sub.2 --;
• R.sup.4 is H, or R.sup.4 +R.sup.6 is selected from --CH.sub.2 --.sub.2 and --CH.sub.2 --.sub.3 ;
• R.sup.5 and R.sup.6 are alkyl groups of 1 to 5 carbon atoms, or together are selected from --CH.sub.2 --.sub.4 and --CH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 --;
• R.sup.7, having a total of up to 8 carbon atoms, is selected from trimethylene and tetramethylene groups, unsubstituted, or substituted with 1 to 4 alkyl groups.